Potable Water Supply Viability for Building Site, Carate, Costa Rica

For:  Cheryl Chip

By:  Osa Water Works

May 11, 2003

Introduction

          Figure 1 is a property boundary map showing the extent of the Carate property purchased recently by Cheryl Chip from Warren and Cindy, who owned the property for several years but never completed development plans.  The authors of the present investigation performed a driveway survey for the previous owners in 1985.  Water supply was attempted by the previous owners, but according to Elías the current property cuidador, these efforts were done on the cheap and were never completely successful.  A beautiful building site is located on a knoll on the property with a breathtaking view of the entire Pacific coastline.  A driveway has been cut to the top of this knoll and the first 200 meters or so have been cemented in.  Beyond this, the property is undeveloped.

Figure 1.  Property Boundary map, showing the building location.

Findings and Discussion

Figure 2 is a topographic map showing the location of the building site and the stream located along Shady Lane, including tributaries feeding this stream.  The Shady Lane stream does not have year round flowing water.  Moreover it is located at an elevation significantly lower than that of the building site, meaning that delivery of water that is available in this watershed would necessarily require energy investment in order to pump the water to the knoll.

Figure 2.  Topographic map showing location of building site (yellow square), proposed water source (blue circle), and the proposed trajectory of the pipeline (blue curve).

The nearest source of water occurring at an elevation sufficiently high to deliver water by gravity to the knoll is shown in Figure 2 as a blue circle.  This potential water intake location is situated on property owned by Mario Urpí.  This particular section of his property is designated as common-use reserve property.  Property owners that purchase from Mario Urpí have access to this and subjacent reserve areas for the development of domestic water supply.  Despite the fact that the property in question was not acquired from Mario Urpí, the previous owners did have permission for water development in this watershed, and their crude water intake is still recognizable in the watershed as well as the litter of their abandoned and ruptured pipeline.  Due to the fact that this property has historically possessed the permission to extract water, the current owner has a de facto prerogative to preserve this permission and to improve the water supply for current purposes.

As the map trajectory reveals, collection of surface water at an elevation of 100 meters is capable of delivery to the 70 meter high knoll with about 30 meters of head to spare for overcoming frictional losses in the pipeline capable of supplying adequate domestic water pressure to a 1-3 story structure built there.

The flow rate observed at the recommended water intake location was approximately 60 gpm at the time of the survey.  Standard United States domestic design criteria are based on an assumed daily usage rate of 100 gallons of water per permanent resident.  This criterion is based on mean water-use rates, including allocations for consumption, cooking, cleaning, laundry, dishwashers, car-washing, and lawn irrigation.  In practice, this daily personal consumption criterion greatly exceeds what is required in Costa Rica, unless a substantial irrigation demand is anticipated.  Using the conservative criterion and assuming a peak occupancy of 10 persons, water demands for the residence under consideration correspond to 1,000 gallons per day, which is equivalent to less than 1 gpm, slightly more than 1 percent of the actual water flow available at the driest time of the year.  In practice, actual water withdrawal rates are likely to be significantly lower unless extensive irrigation is planned for the summer months.  The preceding analysis presumes the use of a storage tank to provide water as needed.  After evaluation, it is recommended that in this particular application a storage tank be considered an optional feature, due to circumstances described in the following section.  Without a storage tank, peak water withdrawals from the watershed under routine domestic consumption are expected to be around 20 gpm.

The analysis reveals that sufficient water can be delivered year round to the knoll to satisfy a well-anointed household.  The proposed location for the water intake is about 50 meters upstream from the only other water intake along the stream.  This lower water intake services a residence recently sold by Bruce Linn to a new landowner whose name is unknown to the authors.  The expected extraction rate is not expected to adversely impact the lower water intake nor to remove the necessary environmental allocation of stream water in the upper watershed. 

Water Quality Considerations

          During OWW’s history of water quality characterization, we have never tested a surface water source that was not contaminated with fecal coliform bacteria.  Fecal coliform is an indicator organism that can exist only in the intestinal tract of warm blooded animals.  While fecal coliform in itself is not a bacterial pathogen, it is a universally recognized “indicator organism” of other enteric organisms that are disease-causing in human beings.  Due to the pristine nature of the watershed, it is inconceivable that human pathogens (typhus, hepatitis, cholera, typhoid fever, bacterial dysentery, etc.) represent a health threat.  However, the protozoan giardia lamblia is a pathogen carried in the intestinal tract of wild mammals.  It is a known pathogen in the Costa Rican forest and causes a severe form of amebic dysentery popularly known as “beaver fever,” among other things.  Protection of a potable water supply from this and other potential but far less common pathogens can be achieved only through disinfection of water supplies.  OWW has documented that our proprietary intake structure (infiltration gallery) achieves an average 85% reduction in fecal coliform content.  OWW has a modular water filtration system that achieves 100% reduction of the remaining bacteria through ultraviolet disinfection.  The owner will want to consider such technology for a whole-house water purification system once construction has been completed.

Design and Installation Budget

The configuration of the water source and the building site makes for a simple design, consisting of nothing more than an infiltration gallery as an intake structure in the stream and a 1,250 meter 2” PVC pipeline for delivery of this water to the building site on the knoll.  A six-meter high water tower is an optional supplemental installation that would be needed only to boost water storage in the event that large water usage rates are expected for irrigation or to provide for emergency water storage in the event that pipeline repairs are needed due to a fallen tree or branch.  In practice, given the limited extent of the area surrounding the building site, it is probable that a direct line will provide for both domestic and modest irrigation water demands without the need for the supplemental water made possible by the storage tank.  Nevertheless it is always desirable to have emergency water storage on hand for unforeseen developments.

The proposed pipeline design provides for a static water pressure of 42 psi and a running pressure of 27 psi at a flow rate of 20 gpm (four people showering simultaneously using normal showerheads, i.e. not water-conservation fixtures).  As a point of comparison, typical United States municipal systems provide 40 psi of water pressure, and the Puerto Jimenez municipal water supply provides 12 psi.  Water pressure tanks used in conjunction with domestic wells typically kick on when pressure drops to 20 psi and cut out when a pressure of 40 psi is attained.  Therefore, a direct line in this application will provide domestic pressure comparable to United States standards.  A 6 meter water tank on the property, on the other hand, would provide a water pressure of only 9 psi.

Table 1 contains a breakdown of the costs of materials, labor, transport, and miscellaneous supplies for the installation of a maintenance-free water supply system that provides for domestic water pressure of 27 psi.  The price for the installation of a water tank for supplemental water storage is also provided as a footnote to Table 1.  Pipeline for this installation would be buried whereever possible to be invisible from the surface environment.  The work also includes removal and disposal of the substantial amount of refuse and litter that was left in the watershed by the former owners.

Table 1.  Installation budget

Timeframe and Terms

The work will require one week for planning and materials acquisition and three weeks of actual work.  Contractual terms for OWW’s undertaking the work are 65% deposit, the balance due upon project completion.